Implantable electrical signal generators, such as pacemakers, defibrillators, neurostimulators, and the like, have been used to treat a variety of diseases. Such devices generate electrical signals that are transferred to a patient's tissue through electrodes disposed on a distal end portion of a lead. The proximal end portion of a lead typically contains a number of connector rings corresponding to the number of electrodes. Conductors run within and along the lead body and electrically couple the connectors to the electrodes. The proximal end portion of the lead is inserted into lead receptacle of a signal generator such that electrical contact is made between discrete contacts in the connector portion and the connector rings of the lead. The lead is then typically secured within the lead receptacle of the signal generator via a set screw, which provides a compressive force on the lead, typically at one of the connector rings. In such cases, the set screw and associated lead retention assembly serve to not only retain the lead within the receptacle but also provide an electrical connection with the signal generating electronics of the device.
When the set screw and associated lead retention assembly are used for purposes of electrical coupling of a lead to signal generating electronics of the device, care is taken to prevent inadvertent stimulation of tissue in the vicinity of the set screw. Such “pocket stimulation” is typically prevented by electrically isolating the set screw from tissue of the patient in which the device is implanted. Electrical isolation is often achieved by placing a silicone grommet between the set screw and the outer surface of the device. The grommet includes a slit to allow a screw driver access to the set screw. However, if the grommet tears or rips during the process of tightening the screw, the electrical insulating properties of the grommet are compromised.
Others have proposed using a plastic set screw or a composite set screw with a plastic head to prevent pocket stimulation, where the nonconductive plastic portion of the set screw serves to electrically insulate the conductive portions of the set screw or conductive portions of the lead retention assembly from tissue of the patient. However, plastic set screws or set screws having plastic heads may strip more easily than their metallic counterparts. If the set screw strips, it may not be adequately tightened to retain the lead or may not be able to be removed should a lead replacement be desired.